From what I have read, the jack demseys are supposed to be somewhat aggressive. I have 2 jacks in a 100 gal. tank with 2 oscars and a large pleco, and the oscars seem to be more aggressive in the tank. I love all of them, but the jack dempseys are much more beautiful IMO. They aren't as trusting as the oscars. I could hand feed the oscars, but the JD's won't come as close. They don't seem to do as much rearranging of the tank as the oscars do either.
if your talking about adding fish to the 55 I'd have to say Dempseys. They will (or should) stay under 12" (the width of the 55) So one pair at full grown will work, get a group of 6 and grow them out and pick a Pair when they are 3-4". I have a jack and I've had Oscars and I feed them anything and everything. if its made for fish food most likely they will eat it.